The invention relates to a method for operating a wiper drive having a motor, a computer program product comprising a program code, which is stored on a machine-readable carrier, and a control device for a wiper drive. The invention particularly relates in this case to the load characteristic of the motor torque during the wiping operation.
Numerous methods for controlling wiper drives (WSA) are known. For example, The German patent specification DE 101 44 985 describes a drive, which drives a wiper by means of a wiper shaft and a torque dependent on the position of the transmission. A control is used here such that the torque delivered by a motor is ascertained at any moment while taking the transmission ratio into consideration.
In wiper drives, the components are subjected to loading by forces. Said forces then increase in wiper drives, as said drives are, for example, used in motor vehicles for operating windshield wipers, in the case of snow load, wind load or when the wiper is blocked. Especially in the latter case of blockage, in particular when the systems are frozen, very large loads can occur, which damage or destroy the wiper system. This can lead to the wiper system breaking down as a result of component failure.
Drives comprising reversing electric motors, in which the motor changes rotational direction in order to generate the reciprocating motion of the wiper, are typically used in wiper systems having direct wiper drives. The wiper blade thereby changes the direction of motion thereof at a lower or respectively upper turning location. The use of reversing motors of this kind further allows for an extended park position (EPS), i.e. a park position below the lower turning location. When switched off, the wiper can thus be parked in a very low position, which is not reached by the wiper system during normal operation.
As a protective function against damage or destruction, a reduced voltage and consequently a reduced motor torque for reducing load can be delivered to an electric motor contained in a wiper drive by means of pulse width modulation (PWM). During operations, the wiper motor can thereby be reduced in the torque thereof in critical ranges. This has, however, a limit due to a reducible voltage value for the drive unit, wherein the voltage cannot be arbitrarily reduced. In simple systems, the speed of the drive and thus the speed of the wiper can furthermore be reduced in the region of the turning locations. The characteristic curve for the pulse width modulation for the wiping operation is laid out during normal operation such that the wiper system can fully traverse the range thereof and remain operable even at high wind speeds. The minimum required rotational speed of the wiper system, which is prescribed by law and according to which a predefined rotational speed may not be undershot in the lowest stage of said wiper system, is particularly to be taken into account in this instance. In the event of the wiper system being blocked, a sufficient level of sensitivity should however be provided to protect the system. The statutory regulations conflict however with the basic idea of the protective function for blocking the system because the required rotational speeds can possibly no longer be provided when a corresponding reduction in motor torque occurs.